Fluid meters such as, for example, densitometers, volumetric flow meters, and Coriolis flow meters are used for measuring one or more characteristics of substances, such as, for example, a density, a mass flow rate, a volume flow rate, a totalized mass flow, a temperature, and other information. Vibrating fluid meters include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations. The one or more conduits provide a primary containment of the measured fluid. The measured fluid may comprise a liquid, a gas, or a combination thereof. The fluid may include suspended particulates.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. The one or more conduits are vibrated by at least one driver at a resonance frequency in one of these modes, hereinafter referred to as the drive mode, for purposes of determining a characteristic of the substance. One or more meter electronics transmit a sinusoidal driver signal to the at least one driver, which is typically a magnet/coil combination, with the magnet typically being affixed to the conduit and the coil being affixed to a mounting structure or to another conduit. The driver signal causes the driver to vibrate the one or more conduits at the drive frequency in the drive mode. For example, the driver signal may be a periodic electrical current transmitted to the coil.
One or more pick-offs detect the motion of the conduit(s) and generate a pick-off signal representative of the motion of the vibrating conduit(s). The pick-off is typically a magnet/coil combination, with the magnet typically being affixed to one conduit and the coil being affixed to a mounting structure or to another conduit. The pick-off signal is transmitted to the one or more electronics; and according to well-known principles, the pick-off signal may be used by the one or more electronics to determine a characteristic of the substance or adjust the driver signal, if necessary.
Generally, the conduits as well as the driver and pick-offs are enclosed within a case. The case can provide numerous benefits, such as protection of the internal components as well as offer a secondary containment of the fluid if the fluid conduits develop a crack, for example. In order for the case to provide adequate secondary containment, the burst pressure (pressure at which a component fails) of the case should be at least as high as the operating pressure of the wetted fluid path (fluid conduits, manifold, flange, etc.). Many of the vibrating meters currently on the market have a wetted fluid path with a burst pressure of around 15,000 psi (1,034 bar); however, this number may vary depending on the material used for the wetted fluid path, the size of the meter, etc. A pressure rating for the wetted fluid path can then be assigned by a regulatory or safety agency based on the burst pressure or some other analytic equation. The secondary containment pressure rating typically includes a safety factor such that the rated pressure is below the actual burst pressure. For example, the American Society of Mechanical Engineers (ASME) currently implements a safety factor of about six to ten, depending on material properties, and the welding methods employed. Therefore, for a wetted fluid path having a burst pressure of around 15,000 psi (1,034 bar), the ASME pressure rating, assuming a safety factor of ten, is only 1,500 psi (103 bar). Due in part to the conservative pressure ratings of regulatory agencies, the burst pressure of the case must also increase drastically to provide approved secondary containment. This extreme increase in the case's burst pressure is problematic, especially when considering that the diameter of the case will always be much greater than the diameter of the wetted path components.
In order to understand how to increase the pressure rating of the case, the shape of the case can be simplified and characterized as a thin-walled, cylindrical-shaped component where the pressure within the case acts against the walls of the case creating a hoop stress. Hoop stress can be characterized by equation (1).
                    σ        =                              P            *            ID                                2            ⁢            t                                              (        1        )            
Where:
σ is the hoop stress;
P is the internal pressure;
ID is the internal diameter of the case; and
t is the case thickness.
Other stresses also exist, such as an axial stress, however hoop stress is typically the largest and therefore the most relevant to choosing a minimum thickness for a desired pressure rating. In many situations, the maximum allowable hoop stress is governed by regulatory agencies or other safety standards. Although the stresses experienced by the case are in reality more complicated than depicted by equation (1), the equation can provide a basic understanding of the main forces acting on the case. As can be appreciated from equation (1), one approach to maintaining an acceptable hoop stress while allowing for a higher pressure would be to decrease the internal diameter of the case. However, this approach is rarely possible without also decreasing the size of the fluid conduits. Another approach would be to increase the case thickness. The case is often formed from a metal such as stainless steel or carbon steel; although, other materials may be used, such as plastic. In relatively smaller meter sizes, i.e., less than about 1 inch (2.54 cm) internal conduit diameter, the standard case is often strong enough to provide adequate secondary containment for the fluid or alternatively, providing extra thickness to the steel case is reasonable and relatively inexpensive. As can be appreciated, as the conduit diameter increases, the case size typically also increases. Consequently, in vibrating meters that include conduit sizes greater than an approximately 1 inch (2.54 cm) internal diameter, the case's ability to contain the fluid pressure upon a conduit failure is diminished and increasing the thickness of the case has serious drawbacks. For example, some large flow rate vibrating meters can have cases with an internal diameter of 10 inches (25.4 cm) or more. Meters of this size are often seen in the oil and gas industry where secondary containment is becoming more important. Cases of this dimension often have a burst pressure of around 860 psi (59.3 bar), many times below the wetted path burst pressure of 15,000 psi (1,034 bar). With dimensions of this magnitude, the case would require a thickness of about 2 inches (5.08 cm), resulting in a case weight of over 2,000 pounds (908 kg) in order to have a burst pressure of 15,000 psi (1,034 bar). As can be appreciated, such an approach results in an excessive cost and weight for the vibrating meter case.
Due in part to the high cost and weight associated with increasing the thickness of the case, the cases used in the prior art for these larger meters were provided simply to protect the conduits and electrical components of the vibrating meter, but did not provide acceptable secondary fluid containment. This created a situation where a conduit failure would almost immediately result in a case failure. Due to ongoing safety concerns in addition to recent oil spills, chemical spills, and environmental concerns, there is increased demand to ensure that the cases of vibrating meters provide a secondary containment if a fluid conduit fails.
The embodiments described below overcome these and other problems and an advance in the art is achieved. The embodiments described below provide a sensor system with a case support surrounding at least a portion of a case of a sensor assembly. The case support can dramatically increase the burst pressure of the case while minimizing the added weight and effect on the case's vibrational frequencies. In some embodiments, the weight added by the case support can be reduced by forming the case support with a plurality of ribs extending from a center portion of the case. The ribs can increase the burst pressure of the case while minimizing the weight added.